706 REPORT — 1903. 



regularly in certain directions, as if solely caused by the regular rotation during a 

 long series of years of the magnetic poles round the geographical poles, for if you 

 examine Map No. 1, showing the results of observations during the years 1840-80 

 as regards secular change, you will observe that there are local causes at work in 

 certain regions, whilst in others there is rest, which must largely modify the eifect 

 of any polar rotation. 



Allow me to explain fui'ther. The plain lines on Map No. 1 indicate approxi- 

 mate regions of no secular change in the declination, and the small arrows the 

 general direction (not the amount) in which the north-seeking end of the horizontal 

 needle was moving during those forty years. The foci of greatest change in the 

 declination, with the approximate amount of annual change in the northern hemi- 

 sphere, are shown in the German Ocean and N.W. Alaska, in the southern 

 hemisphere off the coast of Brazil, and in the South Pacific between New Zealand 

 and Cape Horn. The two foci of greatest annual change in the dip are shown, 

 one in the Gulf of Guinea where the north-seeking end of the needle was being- 

 repelled strongly upwards, the other on the west side of Tierra del Fuego, where 

 the north-seeking end of the needle was being attracted strongly downwards. 



It is remarkable that the lines of no change in the declination pass through the 

 foci of greatest change in the dip. If the needle be repelled upwards, as at the 

 Gulf of Guinea focus, it will be found to be moving to the eastward on the east 

 side of the whole line of no change in the declination from the Cape of Good Hope 

 to Labrador ; to the westward on the west side. If the needle be attracted down- 

 wards, as at the Tierra del Fuego focus, it will be found moving to the westward 

 on the east side of the whole line of no declination from that focus to near 

 Yancouver Island ; to the eastward on the west side. 



A similar result may be seen in the line passing through a minor focus of the 

 dip near Hong Kong. 



Judging from analogy there should be another focus of change in the dip in 

 lat. 70° N., long. 115° E., or about the position assigned to the Siberian focus of 

 greatest force. 



On Map No. 2 are shown lines of equal value of the declination — the red lines 

 for the vear 1880, the black lines for the year 1895. From these, when shown on 

 a large "scale, we may deduce the mean annual change which has taken place in 

 the declination during the fifteen years elapsed. 



In this map we are reminded of the different results we obtain in different 

 localities, for if a line be drawn from Wellington in New Zealand past Cape York 

 in Australia to Hong Kong, little or no change will be found in the neighbouring 

 region since 1840. Again, the line of no change in the declination shown on 

 Map No. 1 to be following much the same direction as the great mountain ranges on 

 the west side of the American continent, has hardly moved for many years accord- 

 ing to the observations available. 



On the other hand, let us now turn to an example of the remarkable changes 

 which may take place in the declination unexpectedly and locally. The island of 

 Zanzibar and the east coast of Africa were constantly being visited by our survey- 

 ing-ships and ships of war up to the year 1880, observations of the declination 

 being made every year at Zanzibar during the epoch 1870-80. The results showed 

 that from Capetown nearly to Cape Guardafui the annual change of that element 

 hardly exceeded 1 . 



During the succeeding years of 1890-91 observations were made by the Ger- 

 mans at Dar-es-Salaam and some other places on the neighbouring coasts, with the 

 result that the declination was found to be changing at first 3' annually, and since 

 that period it had reached 10' to 12' at Dar-es-Salaam. Subsequent observations 

 at the latter place in 1896-98 confirmed the fact of the great change, and in addi- 

 tion our surveying-ship on the station, specially ordered to ' swing ' at different 

 places in deep water off the coast, generally confirmed the results. It is remarkable 

 that whilst such great changes should have taken place between Capetown and 

 Cape Guardafui, Aden and the region about the straits of Bab-el-Mandeb seem to 

 be comparatively unaffected 



